The present invention relates to a light modulator for a display for the presentation of two- and/or three-dimensional image contents. The present invention further relates to a display and to a method for operating a light modulator.
LC Alignment Layers
Conventional liquid crystal displays (LC displays) are typically based on an equilibrium of an interaction of the LC molecules with a surface and an interaction with an electric field.
A surface interaction defines a preferred orientation of the LC molecules during the absence of an electric field. If an electric field is applied, then a force is exerted on the molecules which is usually opposing the surface orientation or which is effective in a predetermined direction. Depending on the electric field strength and on the elastic forces among the LC molecules, the LC molecules will rotate into a resultant orientation. There is typically a threshold field strength above which the orientation of the LC molecules will not change compared with the situation without field. Further, there is a saturation field strength, which means that in fields which are stronger than that saturation field strength the orientation of the LC molecules will change no further. Between said threshold field strength and said saturation field strength, the LC orientation will change along with the field strength, in particular continuously.
In a conventional LC display, such as in a PC monitor, this variable orientation serves for the representation of greyscale values in dependence on the control voltage of an LC pixel.
As far as defining the surface interaction is concerned, there are a number of methods known in the prior art. As a standard, a polyimide layer is used in LC displays which is given a preferred orientation by mechanical rubbing or brushing. At the interface with the polyimide layer, the LC molecules align such that their long axes are parallel to the rubbing direction. The strength of the surface interaction can be varied by accordingly choosing certain parameters during the manufacturing process, e.g. by applying different mechanical rubbing pressure.
Depending on the type of LC display, one of these three orientations is required: a planar orientation where the preferred orientation of the LC molecules is parallel to the surface of the glass substrate, a homeotropic orientation, where the preferred orientation of the LC molecules is perpendicular to the surface of the glass substrate or a tilted orientation where the preferred orientation of the LC molecules has a defined angle to the surface of the glass substrate. A planar orientation is for example typically used in an in-plane switching (IPS) display, whereas a substantially homeotropic orientation is for example used in a vertical-alignment (VA) display.
Certain types of LC materials—for example nematic LCs with a positive dielectric anisotropy—preferably align such that their long axes are parallel to an electric field. Other LC materials—for example nematic LCs with a negative dielectric anisotropy—preferably align such that their long axes are perpendicular to an electric field.
A certain combination of surface orientation on one substrate or on either substrate with a type of LC material and an arrangement of electrodes will be referred to below as an LC mode; so for example the combination of planar orientation on either substrate, nematic LCs with positive dielectric anisotropy and electrodes which are arranged such that they generate an in-plane field is referred to as IPS mode.
Now, a surface interaction could be defined with a number of different materials and using various methods. In addition to the method of rubbing polyimide, which has already been mentioned above, it is further possible to use a material in the form of silicon oxide films which are vapour-deposited, where the parameters of the vapour deposition process are chosen such that a suitable preferred orientation of the LC molecules is achieved. Moreover, a light-sensitive polymer material could be given a preferred orientation by way of optical methods e.g. using UV light (photo-alignment), where that preferred orientation serves to align the LC molecules accordingly. Publication [1] provides an overview of alignment layers which are known in the prior art.
Usually, the entire display panel of an LC display is given a uniform and fix preferred orientation. However, a spatially structured preferred orientation could be provided alternatively which makes it possible to realise for example different orientations of the LC molecules for each pixel.
Still further, switchable preferred orientations are known in the prior art: Blinov/Chrinov describe in their book “Electrooptic Effects in Liquid Crystal Materials”, under the headline “Multistable Orientation”, p. 125 [2], that for a nematic liquid crystal in contact with a crystalline substrate which comprises an n-fold rotational symmetry of the crystal, where n is an integer number, the LC molecules can generally have n different orientations along one of these directions defined by the crystal. Individual domains can then form with different orientations along one of these n directions. An external in-plane field can switch the orientation in individual domains to a different one of the n possible directions. It is further described that a multistable orientation can also be achieved with the help of vapour-deposited silicon oxide films. An electric field can then serve to switch from one of the stable orientations to another one. That text further describes the concept of a bistable LC display which is based on the switching of a nematic LC between two possible surface orientations. If no electric field is present, then the molecules remain in one of the possible preferred orientations. This makes the display similarly energy-saving as a bistable ferroelectric LC, because no power is required to display unchanging content.
Further, publication [3] describes a tristable LC device which is based on a finely-patterned surface structure. A very fine spatial pattern of different preferred orientations is written to a polyimide surface. The microscopic pattern causes three equally stable LC orientations to be realised macroscopically. An electric in-plane field serves to switch between them.
The photo-alignment technique could also be taken advantage of for optically switching between the surface orientations, e.g. such that photosensitive molecules in the surface layer of a polymer material change their conformation under the influence of UV radiation, thereby inducing different preferred orientations of the LC molecules which are attached to this surface layer of the polymer material. This way, the LC orientation can be switched through the surface interaction in an optically addressed spatial light modulator (OASLM). This photo-alignment technique can for example also serve to switch from a planar to a homeotropic orientation of the LC molecules. A disadvantage of optical switching is its low speed, so that it is hardly suitable for use in a light modulator which is supposed to work at high refresh rates.
There are different liquid-crystal phases. In nematic liquid crystals, a uniform orientation of molecule axes is energetically favoured. However, the position of the individual molecules is distributed statistically. In smectic liquid crystals, the molecules additionally find together in layers. A special type of smectic liquid crystals, the chiral smectic C phase (SmC*) comprises ferroelectric properties. Due to the hysteresis which is caused by ferroelectricity, this type also allows a bistable display to be realised. A certain voltage threshold is necessary to switch the LC molecules into one state. They will remain in this state until a voltage with opposite sign and absolute value which is also above said threshold is applied. Then, they will switch to the other state. During this, an out-of-plane electric field is applied. However, the LC molecules rotate in a plane which lies parallel to the substrate. In addition to ferroelectric LCs (FLC), there are further phases with similar properties, for example anti-ferroelectric and ferrielectric. Ferroelectric LCs are characterised by their high switching speed. However, they have other disadvantages. Moreover, arrangements have been described which comprise SmC* LCs but which do not comprise hysteresis and thus no bistability, but a continuous variation in the LC orientation as the applied voltage is changed, which is referred to as the V-shaped FLC mode.
Publication WO 00/03288 A1 describes an electro-optical device whose surface layer itself has liquid-crystal—or, more precisely, chiral smectic—properties. That device additionally comprises a bulk LC layer which can for example also be a nematic LC.
The orientation of the liquid crystals in the surface layer can be switched by applying an electric field (“primary surface switching”). That surface layer then induces a certain orientation of the bulk layer (“induced bulk switching”). For example, the LCs in the bulk layer can have a preferred orientation that is parallel to that of the LC molecules in the surface layer. An additional fix separate (conventional) surface layer can be provided which determines the absolute orientations in order to align the smectic LC molecules of the dynamic surface layer in relation to that separate layer in their bistable switching states. A controllable surface layer will also be referred to as alignment means below.
The publication highlights the high switching speed of the smectic LCs in the surface layer as the major advantage. The idea is to achieve shorter response times for example of nematic LCs in the bulk layer thanks to the induced alignment of the bulk layer than would be achievable when controlling of the bulk layer directly.
The publication also includes an embodiment in which an alignment of the bulk layer which is induced with the help of the surface layer can be combined as a two-stage effect with a direct alignment of the bulk layer which is realised with the help of an electric field. In that embodiment, the threshold of the electric field strength for direct alignment of the bulk layer is higher than the field strength which is required for switching the surface layer. A change in the orientation of the molecules of the bulk layer in the plane which is parallel to the substrate can thus be induced by applying a low out-of-plane electric field below the threshold field strength and thus by switching the surface layer. A stronger out-of plane field which is applied to the same electrodes controls the bulk layer directly such that the LC molecules rotate out of the plane which is parallel to the substrate.
Instead of a separate surface layer which is immiscible with the bulk layer, publication WO 03/081326 describes how a bulk layer is doped with molecules which are distributed in the bulk layer but firmly attached to the surface. The spatial separation of surface layer and bulk layer is thereby overcome, but still an alignment in the bulk layer is induced with the help of the defined orientation in the surface layer. The surface layer can for example include side-chain polymers whose side chain can be rotated, i.e. aligned, in relation to the main chain, while the backbone chain serves to provide the firm connection with the surface.
Phase Modulation
Phase-modulating SLMs are required for coherent optical applications, such as holographic display devices. These are typically light modulators which have pixels that are regularly arranged in the x and y dimension. The pixels are made such to modulate the phase of the light which interacts with the light modulator. The modulation of the light which interacts with a pixel is in particular relative to the modulation of the light which interacts with another pixel. For a given wavelength—for example in the visible range—this requires a phase modulation of between 0 and 2π. Holographic display devices require particularly fast phase-modulating light modulators.
Several types of conventional amplitude-modulating LC displays can be modified easily such to modulate the phase instead of the amplitude, e.g. by changing the polarisation of the incident light. However, the required phase range of up to 2π can hardly be served then; the phase range of such modified display panels is for example only 0 to π.
LC modes such as electrically controlled birefringence (ECB) or vertical alignment (VA) require a combination of layer thickness d and birefringence Δn such to satisfy the equation Δn·d=λ/2 for amplitude modulation, but rather Δn·d≥λ/2 for phase modulation up to 2π, where λ is the wavelength of the light which is to be modulated.
Compared to an amplitude-modulating light modulator, a phase-modulating light modulator which uses one of these LC modes either requires an LC material with a larger refractive index, which often also shows higher viscosity, i.e. which reacts more slowly, or which comprises a greater layer thickness, where conventionally the response time of a nematic LC is about proportional to its squared layer thickness. This is why a phase-modulating light modulator based on those display types would be disadvantageously slower than a comparable amplitude-modulating display.
However, it would generally be possible to accelerate these LC modes by using a dynamic alignment layer while maintaining the range of possible orientations of the LCs. But the slowest switching operations are grey-to-grey transitions in an amplitude-modulating display, or, adapted to phase modulation, the transitions among medium phase shifts. Since there is only a limited number of defined surface orientation states, it must be suspected that not all required phase steps can be accelerated as desired.
ECB and VA in the above-mentioned configuration require linear polarised light for phase modulation.
Another possibility of phase modulation according to Pancharatnam [4] takes advantage of circular polarised light and a controllable λ/2 plate (therefore Δn d=λ/2 applies here). If the optical axis of the λ/2 plate is turned by the angle ϕ in the plane which is parallel to the surface, then a phase modulation of 2ϕ is obtained. An angle ϕ of 180 degrees is required to achieve a phase of 2π.
Similarly, a reflective arrangement can be provided like this: Circular polarised light passes through a rotatably arranged λ/2 plate, then through a fix λ/4 plate, then hits a reflective layer and passes on its way back the fix λ/4 plate and the rotatably arranged λ/2 plate again. The rotatably arranged λ/2 plate can for example be realised in the form of a suitable layer of liquid crystals. The fix λ/4 plate can for example be realised in the form of a polymer film. If the λ/2 plate is turned by the angle ϕ, then the light will undergo on its way to the reflector a phase modulation by twice that angle, i.e. by 2ϕ, and after reflection on the way back another phase modulation by 2ϕ, i.e. altogether by 4ϕ. A total angular range of ϕ=90 degrees is then required to realise a phase modulation of up to 360 degrees (2π).
Another possibility of embodying a reflective arrangement according to the prior art is to use a rotatably arranged λ/4 plate and a reflective layer, where the λ/4 plate is passed twice, namely on the way to the reflector and on the way back, so that it has the effect of a transmissive rotatably arranged λ/2 plate for the light. The rotatably arranged λ/4 plate can for example be realised in the form of a suitable layer of liquid crystals. In that case, the phase modulation that is realised when the λ/4 plate is rotated by the angle ϕ is again twice that rotation angle, i.e. 2ϕ. An angular range of altogether ϕ=180 degrees is again required to realise a phase modulation of up to 360 degrees (2π). Where LC-based phase-modulating light modulators are used, the layer thickness of the λ/4 plate is halved compared with a transmissive modulator, which can have a positive effect on the response time.
Both possibilities for reflective modulators are described in publication [7] and shown schematically in FIG. 2 there.
With LCs, the optical axis typically corresponds with the long axes of the LC molecules. The optical axis can thus be tilted by tilting the LC molecules accordingly in the plane.
Certain types of LC displays, such as in-plane switching (IPS) or polarisation-shielded smectic (PSS), see for example publication US 2007002267 A1, use a layer thickness of Δn d=λ/2 and rotate the LC molecules by controlling them through an in-plane field. PSS-type amplitude displays are particularly fast. Control frequencies of up to 1 kHz and more have been achieved. IPS uses nematic LCs and an in-plane electric field. PSS uses smectic molecules and an out-of-plane electric field. To achieve amplitude modulation, these LC modes conventionally work with linear polarised light. However, it would be easily possible to modify the polarisation state. For example, a λ/4 plate can be used in order to convert linear polarised light into circular polarised light. Nematic and smectic molecules differ in that nematic molecules align in the same direction irrespective of the sign of the applied voltage, while the orientation of certain smectic LC molecules depends on whether a positive or negative voltage is applied.
In a PSS-type phase-modulating light modulator, the direction of rotation and thus the phase depends on the sign of the voltage. Document DE 10 2009 002 987.7 describes the use of the PSS mode in a phase-modulating light modulator using circular polarised light under consideration of the sign of the voltage. However, the conventional rotation angle of the LC molecules in the IPS and PSS modes does not suffice to realise a phase modulation of 2π.
In a transmissive PSS-type amplitude-modulating light modulator and with suitable linear polarisation of the incident light, there will be a transmittance profile which is proportional to the squared sine of twice the rotation angle 2ϕ.
Minimum transmittance is achieved for ϕ=0 degrees, and maximum transmittance is achieved for ϕ=+45 degrees or ϕ=−45 degrees. Ideally, a transmissive PSS-type amplitude-modulating light modulator uses a rotation angle range of between +45 degrees and −45 degrees. In that angular range, it would also be possible to use a reflective phase-modulating light modulator with rotatably arranged λ/2 plate and a fix λ/4 plate.
However, the useful range of rotation angles ϕ for PSS is limited by the tilting angle of the LC molecules in the smectic phases which are used for PSS (in particular smectic C). This tilting angle is a material-specific parameter which depends on the actually used liquid crystal substance and which also fluctuates with the temperature. Since nematic materials are frequently used in conventional LC displays, major LC manufacturers offer mostly nematic LC materials. The choice of commercially available smectic LC materials is much smaller.
This is why the angular range of between −45 degrees and +45 degrees can often not be realised with the available LC materials or only in small temperature ranges. For example, the achievable angular range is only −35 degrees to +35 degrees.
Another limitation can be that although an angle of 45 degrees can be obtained theoretically the voltage that is necessary to achieve such an angle is too high, i.e. it would exceed the range that is realisable with a conventional LC backplane.
The disadvantages of an angular range that is too small have differently strong effect on amplitude modulation and phase modulation. In the case of amplitude modulation, an angle of 35 degrees results for example in 88 percent of the maximum transmittance. This means that an amplitude-modulating display can also be operated with this angular range if the minor disadvantage of a slightly lower luminous efficacy and consequently a slightly higher power consumption of the display is accepted. In a reflective phase-modulating SLM, however, rotation angles of between −35 degrees and +35 degrees would only allow a maximum phase modulation of 1.55π instead of the required modulation of up to 2π.
If such a phase-modulating SLM is used in a holographic display device, this would have adverse effects on the quality of a holographic reconstruction. This represents a serious drawback.
Amplitude modulation with the IPS mode requires a rotation angle of between 0 and 45 degrees. This will effect a rotation of the linear polarised light by angles of between 0 and 90 degrees, which suffices to achieve either full transmittance or complete extinction with a fix polariser at the exit. Amplitude modulation with the PSS mode takes advantage of angles of between −45 degrees to +45 degrees, where positive and negative angles bring about same amplitudes.
Transferred to phase modulation, this angular range corresponds with phases of between 0 and π/2 (IPS) and 0 to π (PSS) in a transmissive phase-modulating light modulator or a reflective phase-modulating light modulator with an embodiment of the LCs as a rotatably arranged λ/4 plate. In order to achieve the desired phase modulation of up to 2π, the angle at which the LC molecules are aligned had to be enlarged, namely fourfold for IPS and doubled for PSS. This is hardly possible. The maximum rotation angle which can be achieved in a conventional IPS-type display is 90 degrees ±90 degrees would represent a theoretical limit for PSS, which is, however, never achieved in practice.